(1) Field of the Invention
The present invention relates generally to fluid clarification systems and, more particularly, to a system and method of selectively separating undesirable solids from a fluid, particularly drilling mud, while retaining certain desirable solids in the fluid so that the fluid can be subsequently used. Further, the present invention relates to a fluid clarification system which is controlled for maximum efficiency of the removal of undesirable solids from the fluid.
(2) Description of Related Art
The present invention provides a fluid clarification system which may be used with a drilling rig. When an oil well is drilled, it is necessary to drill the well with drilling fluid, commonly referred to in the art as drilling mud. The drilling mud is provided to lubricate and cool the drill bit and to carry away cuttings as the mud flows upwardly in the annular flow space around the drill string. The drilling mud is pumped down the drill string to pick up the cuttings and other debris. Commonly, the drilling mud is water but it is sometimes made with an oil or oil-based carrier.
Generally, various heavy metal or other minerals are added to drilling mud to give it a selected weight and viscosity. The viscosity is obtained from clay or clay products. The drilling mud becomes slick to the touch so that it provides a lubricating benefit.
When drilling into a high pressure formation, safety is enhanced by incorporating a weight component, such as barium sulfate, barite, or hematite, for example, to the drilling mud. Water has a weight of about 8.4 pounds per gallon. The weight of the drilling mud can be increased to as much as 17 or 18 pounds per gallon by adding the weight materials. Occasionally, higher weights are achieved by addition of these or other weight materials. The weight materials may have a relative density of around 4.0 compared to water which has a density of 1.0.
While circulating through the well, drilling mud picks up particles of the earth formations cut by the drill bit. It is relatively easy to clean the drilling mud if the cuttings are primarily heavy rock. Also, large particle cuttings are easily removed from the mud by passing the drilling mud through a set of screens. In general, as mud is returned to the surface, it typically flows into a mud pit and then is pumped out of the mud pit by a mud pump. While flowing from the well to the mud pit and then back to the mud pump, the mud typically is treated by a number of devices to restore the mud to its original condition, such devices including shale shakers, desanders, degassers, and other cleaning devices.
At times, the mud will simply be permitted to sit in an open pit. This enables the heavy particles in the mud to settle to the bottom. Gas bubbles also are removed so that entrained gas bubbles do not create a risk of explosion by accumulating odorless natural gas around the mud pits. Drilling mud with such entrained gas is also too light for almost all applications.
In many ways, separation techniques applied to drilling mud run into problems because of the separation of the desirable added components along with the undesirable components retrieved from the well. As previously described, drilling mud returning from down hole comprises a fluid such as water or a synthetic oil, high gravity materials added to the drilling mud, and low gravity solids (i.e., cuttings) from the drilling operation. Sometimes, depending on the nature of the formation penetrated, the mud will be commingled with cuttings from sand and shale formations (a specific gravity of about 2.6). Sand cuttings are relatively easy to remove. Shale cuttings, having a smaller particle size, are more difficult to sort or separate and cannot be wholly removed by sieves or screens. Moreover, cuttings from clay formations are dissolved into the solution of the drilling mud so that no amount of mechanical screening or filtration can remove them. Operators typically maintain low gravity solids in the drilling mud at 5-6% volume percent of the drilling mud. If undesirable components cannot be removed from the drilling mud, then either the drilling mud must be replaced or diluted with more drilling fluid. Either solution to the problem is quite expensive.
In drilling a well, and especially a deep well, the problems just described are minor at shallow depth and become more and more significant with depth. Typically, the first several hundred feet of drilling will be accomplished in just a day or so and the borehole is drilled rather rapidly. The problem arises at greater depths where the drill bit penetrates several formations of shale. The clay that is in the shale will dissolve, thereby changing the physical characteristics and performance of the drilling mud. Mud will no longer exhibit the integrity necessary for continued reuse. As the drilling mud is adulterated with added well bore materials, it ultimately is necessary to dispose of the entire batch of mud. At that point, the well is quite deep and the amount of mud required for replenishment can be as much as 2000 barrels of fluid. This is expensive with a water based mud and even more expensive with an oil based mud. Some drilling fluids cost as much as $300 per barrel in 1998 prices. It is not uncommon to have as much as $1,000,000 worth of drilling fluid solvents mixed into the drilling fluid and in circulation in a well. It is therefore desirable to extend the useful life of drilling mud as long as possible by removing cuttings and dissolved undesirable components from the drilling mud while retaining the high gravity additives in the mud.
Thus, there is a direct economic benefit in removing as much of the undesirable solids from the drilling mud while retaining the additives in the mud. The natural inclination of operators of clarification systems in the field is to maximize the flow rate of drilling mud through the system. However, running the system at maximum flow rate does not necessarily remove the greatest amount of the cuttings. So, there remains a need for a system with installed controls to operate the system for the maximum efficiency in the removal of the cuttings from the drilling mud. Further, there remains a need for a system which demonstrates the cost savings to the operator if the system is operated at such a maximum efficiency operating point.
The present invention provides a method and apparatus for selective removal of such undesirable low gravity components from the return stream of the drilling mud for maximum efficiency in removing these undesirable components. The apparatus is preferably adapted to be mounted on a skid and installed at a drilling rig. It is preferably skid mounted for ease of transport to and from a work site.
The apparatus of the present invention receives mud returned from the well borehole. The mud is transferred by a pump to a tank and then is delivered from the tank through a first centrifuge. The first centrifuge removes the heavier components and provides a xe2x80x9csolidsxe2x80x9d discharge and a liquid discharge in the manner well known in the art. The solids discharge from the first centrifuge comprises the heavier particles from the drilling mud which are delivered from the first centrifuge in a wet slurry of about 40% solids and 60% fluid. While some drying does occur, the system is operated so that significant and substantial recovery of all the expensive weight material is removed from the mud. In situations where the fluid is an oil based mud, the oil can be recovered also.
The fluid discharge from the first centrifuge, with high gravity components removed but with the undesirable low gravity components still entrained in the mud, is then directed to a second centrifuge. Here, the low gravity components are removed and the second centrifuge provides a xe2x80x9csolidsxe2x80x9d discharge and a fluid discharge. The high gravity components, previously separated by the first centrifuge, are then added back into the fluids discharged from the second centrifuge.
The present invention further provides a system of sensors coupled to a control unit to measure solids content at various points in the system. The sensors and associated control unit determine the amount of solids being removed by the mud processing system and adjust system flow rate for maximum efficiency of the removal of cuttings from the drilling mud.
It is thus an object of the invention to provide an improved, portable, self-contained mud processing system with first and second stage centrifuges. The first stage is operated so that the heavy weight materials of importance are removed. This involves recovering the components of the weight material which have a specific gravity of about 4.0. By judicious adjustment control of the throughput a desirable weight separation is accomplished. The weight materials are recovered substantially free of low gravity components. By using two separate stages, the heavy weight materials of value are removed and placed back in the drilling mud. Whether the drilling mud solvent is water or expensive oil, the present invention permits it to be recycled several times through the mud system.
Moreover, the present apparatus sets out a control so that adequate pump flow is maintained to feed the first and second stage centrifuges and to maximize undesirable solids removal. The centrifuges are provided with a positive pump fluid flow input. In addition, the centrifuges are provided with that input subject to safe control so that the centrifuges are not overloaded. This enables the centrifuges to operate such that each removes a specified or desired specific gravity of solids. The first stage centrifuge removes high gravity solids and the second stage centrifuge removes low gravity solids. The solids are discharged from each centrifuge with a small amount of solvent so that they form a slurry.
The system incorporates a controller which monitors the operation of the pumps and centrifuges to achieve optimum separation. It is thus an object of the present invention to provide a system of sensors and a controller to maximize the removal of undesirable low gravity solids while retaining high gravity additives in the drilling mud.
The system further incorporates a system of mass flow sensors to monitor operation of the system, and to demonstrate an objective measure of the undesirable solids removed by the dual-stage separation system. The central processor receives inputs from the sensors and manually entered data of quantitative analysis of the makeup of the drilling mud at various points in the system. This data is then assembled to provide a readout and a report to demonstrate to a system operator savings realized by use of the system. The data is also assembled to show the quantity of solids and contaminants discharged from the system in order to meet governmental regulations.
The present apparatus is summarized as a skid mounted unit incorporating first and second stage centrifuges. The input is through a mud line connected from the mud pit or other point in the mud system. Storage tanks are also included. The input connects through a first positive displacement pump, then a high gravity solids centrifuge, and then through a second positive displacement pump and then a low gravity solids centrifuge. Following the low gravity solids centrifuge, the solids discharged may then be disposed of or they may be directed to a cuttings drier to further separate drilling mud solvent and low gravity solids.
The system also utilizes appropriate sensors which monitor the state or condition of the two pumps and centrifuges and other system components. Signals are provided to a controller system which monitors operation to avoid system overload and to control system operation for maximum solids removal.